In the coating of photographic layers on a support such as film base or paper, a number of individual layers are coated onto the support simultaneously by means of a multiple slot coating hopper. This process may be performed many times, resulting in a multiplicity of multilayer coatings on the same base. It is often desirable to match the viscosity and density of each of the layers in the multiple layer pack; however, this is not always possible. When the viscosity or density of a layer becomes much different than layer adjacent to it there is a risk of hopper slide wave nonuniformity. Hopper slide waves form because of disturbances to the coating process, such as machine vibrations or flow pulsations. When the viscosity or density of two layers becomes very different these waves can grow substantially resulting in nonuniformity in the final product and significant waste.
In general, in photographic applications densities are very similar. An example of a situation which very often results in a layer viscosity mismatch in the coating of aqueous gelatin based photographic systems is the addition of a diffusible hardener in one of the layers of the last multilayer application. The hardener is designed to react with gelatin to form chemical crosslinks between gelatin molecules in all layers of the plurality of multilayer coatings. This process results in a hardened product with desirable mechanical characteristics for the photographic system. Since this hardener composition is, by design, very reactive with gelatin, the layer which is delivered with the hardener must typically be delivered and coated at a very low gelatin percentage with respect to the solvent, which is typically water, to reduce reactivity. This results in a layer of very low viscosity. It is desirable to coat all layers at as high a viscosity as possible to reduce the severity of nonuniformity due to such things as flow after coating resulting from nonplanar base and flow after coating resulting from air impingement on the coating. It is also desirable to reduce the time needed to chill set a coating so that the coating speed may be maximized. A polymer thickener may be added to increase this viscosity without increasing the hardener gelatin reaction rate. However, this viscosity increase is typically limited by a number of factors including sensitometric shift and changes in layer rheology.
Therefore, very often in practice of the art, the last multilayer application of a photographic product has a low viscosity layer, i.e. the layer which includes hardener, as one of the layers in the pack with the rest of the layers being coated at relatively high viscosities. It then falls upon the engineer to choose which of the plurality of layers is to be the low viscosity layer or layer containing hardener since the hardener can typically be added to any of the layers. An unwise choice of hardener placement may result in a number of coating nonuniformities in the final product. The most important of these nonuniformities are interfacial waves due to strain rate discontinuities at the interfaces between the low and high viscosity layers during flow down the hopper slide. Interfacial waves are formed by disturbances, the most common being hopper vibration, flow rate pulsations, or particulate disturbances.
In many situations, one or more of the layers will have a higher or lower viscosity than the remaining layers due to for example, addition of other chemical addenda. The present invention allows one to place the higher or lower viscosity layer in the position that will produce the most uniform coating. The present invention also provides a method for choosing the layer of a multilayer coating pack to which such viscosity-affecting addenda should be added.